1. Field of the Invention
The present invention relates generally to Kraft recovery furnaces and particularly to black liquor spray nozzles for such furnaces.
2. Description of the Related Art
Kraft recovery furnaces are used to recover useful fuel from the paper making process. In this process logs entering the mill are reduced to chips and are cooked under pressure in a steam-heated aqueous digestion solution of sodium hydroxide and sodium sulfide, known as "white liquor"or "cooking liquor". In the cooking operation, the lignin binder, which holds together the cellulose fibers of the wood, is dissolved.
After cooking, the cellulose fibers, now called "pulp" or "brown stock", are separated from the spent cooking liquor. The pulp may then go through several fiber refining and bleaching processes and finally to the paper machine. The spent cooking liquor containing the lignin dissolved from the wood is called "black liquor". As the diluted or "weak" black liquor (15-18% dissolved solids) comes from the washers, it is first concentrated in a multiple-effect evaporator by the use of steam.
The concentrated or "strong" black liquor then goes to the mix tank where sodium sulfate (salt cake) is mixed with the liquor to make up the chemical losses in the system. Chemical ash recovered from the boiler hoppers and from the fume collector following the direct-contact evaporator is also returned to the liquor cycle. The "heavy" black liquor with its salt-cake burden is heated to lower its viscosity and is then pumped to the recovery furnace. In the furnace, the heavy black liquor is sprayed on the furnace walls for dehydration prior to final combustion of the dried char on the hearth.
The essential function of the recovery unit is the reduction in the furnace of the sodium sulfate content of the black liquor to sodium sulfide.
In the recovery furnace, heat obtained from the combustion of the organic liquor constituents dissolved from the wood is recovered in production of steam, and the inorganic sodium constituents in the liquor are recovered as molten smelt, composed largely of sodium carbonate, Na.sub.2 CO.sub.3, sodium sulfide, Na.sub.2 S.
The spray of the black liquor into the recovery furnace is done by black liquor burners having spray nozzles therein.
There are two designs of liquor burners, the oscillator and the limited vertical sweep (LVS). Both types of burners utilize a nozzle splash plate to produce a sheet spray of coarse droplets.
The oscillator spray controls the black liquor distribution on the furnace walls, where it is dehydrated and falls to the char bed. The oscillator burners typically are located in the center of the furnace wall between the secondary and tertiary air ports, are continuously rotated and oscillated, spraying liquor in a figure eight pattern to cover a wide band of the walls above the hearth.
In the LVS burners shown in FIG. 1, black liquor is sprayed into the furnace for more in-flight drying and devolatization of the combustible gas stream rising from the char bed. The objective of the LVS burner is to minimize the liquor on the wall. The LVS gun is normally used in a fixed position, but can also sweep vertically to burn low solid liquor or those with poor burning characteristics.
The temperature and pressure of atomized liquor directly impacts recovery furnace operations. Lower temperature and pressure generally create a larger particle or droplet of atomized liquor. This minimizes the entrainment of liquor in the combustion gases passing to the heat absorbing surfaces. Where wall drying is carried out, large liquor droplets maximize the liquor sprayed on the wall and minimize in-flight drying.
As the liquor sprayed on the walls builds, it eventually falls to the char hearth. The majority of the char falling from the wall is deposited in front of the primary air ports, requiring 40 to 50% of the primary air to be introduced through the primary ports.
The basic splash plate for these known spray nozzles has been used in recovery boilers. FIG. 1 shows a typical spray nozzle burner with a splash plate nozzle mounted on the burner pipe. FIG. 2 shows a spray gun black liquor distribution pattern from such known burner nozzles.
The concept of splitting the liquor flow in burner spray nozzles is also known. Past, known experimentation used a vertical pin, or cylinder, whose flat end was fastened to the splash plate. Such modified sprayer plates were used on recovery boilers approximately 20 years ago. The embodiment was too erratic in operation and did not provide any benefits and did not receive acceptance in the industry. About the same time, Gotaverken Heat Engineering in Sweden experimented unsuccessfully with a pin mounted on a splash plate.
Thus to date there has not been a successful spray nozzle for black liquor that would split the flow into two opposed flat sheets of black liquor that was controllable without producing splatter and large drop buildup on the walls of the recovery furnace.